Tracy
S. Feldman: Current Research Interests

I study direct and
indirect effects of behavioral and species interactions on population-level
phenomena: When do interactions between species have fitness consequences for
individuals, and when do these fitness effects influence population dynamics? I
am also interested in the ways human-induced changes, such as habitat
fragmentation and introduced species, affect the behavior and population
dynamics of species involved in multi-species interactions (including mutualisms).
In my research, I address these issues using a combination of field and lab
experiments, statistical and mathematical models, and molecular tools. In
addition, I focus primarily on interactions involving fungi, and more recently
viruses, which touch on the rapidly growing but still relatively poorly
developed fields of microbial and virus ecology. I have studied
plant-associated fungi and fungal viruses; indirect interactions involving
plants that share pollinators, or between plants, plant-associated fungi and
insect vectors; multi-trophic interactions between plants; and mutualisms
between plants and pollinators. Thus, I study animal vectors of pollen or
fungal pathogens, and their individual and population-level effects on fungi
and plants. My work addresses three main questions. (1) What factors might
enable plant-pollinator mutualisms to persist when plants are at low densities?
(2) What are the behavioral, fitness, and population consequences of
competitors on mutualisms involving plant pathogens and their vectors? (3) What
role do plant-associated fungal viruses play in ecological communities? To
illustrate my approaches to science, I will describe my work in each of these 3
areas below.

(1) What factors might enable plant-pollinator mutualisms to persist when
plants are at low densities?

Mutualisms may be particularly susceptible to extinction in rapidly changing or
highly disturbed environments. One mechanism that may allow pollination
mutualisms to persist is pollination facilitation, a process by which
reproductive success of a rare plant species increases in the presence of a
second species due to increased visits from shared pollinators. For plants
occurring at low densities, this mechanism might be critical to population
persistence. Thus, through facilitation, plant species may rescue one another
from negative effects of growing at low density, by increasing the quantity or
quality of pollinator visits, and therefore seed production. I explored this
question for my dissertation work (with Dr. W. F. Morris at Duke University),
using a variety of systems and approaches, including mathematical models and
field experiments.

From two separate field
experiments with two different focal plants, Brassica rapa and Piriqueta
caroliniana, I found strong positive effects of increasing plant density on
pollinator visitation (measured as visits to plants per hour and visits to
plants per foraging bout) and reproductive success (seed production). In
addition, evidence from a density-dependent projection matrix model for
population growth in P. caroliniana demonstrates that populations may
grow more slowly at low densities due to reduced seed production, but do not
decline (Allee effects occur but are weak).

To determine when
pollination facilitation might rescue plants from negative effects of low
density, I conducted experiments using Coreopsis leavenworthii, a
co-flowering species that shares pollinators with P. caroliniana. I
found that visitation, pollen receipt, and reproductive success of P. caroliniana
remain unaffected in the presence of the co-flowering species. Thus,
pollination facilitation may not occur when plants simply share pollinators and
co-occur at low densities—other criteria are important. Evidence from a
mathematical model I developed (with Dr. W. F. Morris and Dr. W. G. Wilson)
suggests that one plant species can also facilitate another’s pollination when
the number of pollinator visits to patches of plants per unit time (the
aggregative response) accelerates at low densities. To my knowledge, this
mechanism by which facilitation might occur has not been addressed previously.
However, in a second experiment with Brassica rapa, the pollinator
aggregative response did not accelerate at low densities. Although pollination
facilitation has been found in several systems, and occurs by several different
mechanisms (e.g. through disproportionate increases in visits to individual
plants or through maintenance of larger pollinator populations in more diverse
plant communities), facilitation may be unlikely to occur through increases in
visits to patches of plants.

Combining field
experiments with mathematical models enabled me to use field data to predict
population dynamics under different scenarios. In general, this
interdisciplinary approach is necessary for understanding complex processes
that are difficult to measure directly. In the future, I hope to continue to
address population-level consequences of plant-pollinator interactions,
including other factors that may cause population decline, such as
self-pollination in outcrossing species.

(2) What are the
behavioral, fitness, and population consequences of competitors on mutualisms
involving plant pathogens and their vectors?

In almost all known mutualisms, additional species exploit or compete with one
or both partners, raising the possibility that exploiters could erode the
mutualism over ecological or evolutionary time. However, the consequences of
exploitation for most mutualisms are not well understood. I have been
developing a research project to address this question in the context of a
system of multi-species interactions involving Claviceps, a genus of
fungal pathogens that infect grasses. In the future, I plan to combine
fieldwork, laboratory/greenhouse experiments, and molecular work to study the
effects of a common pathogen (Claviceps paspali) on the fitness of two
invasive grass species (Paspalum notatum and P. dilatatum) that
are hosts to the pathogen. C. paspali can severely damage grain crops
and poison livestock that eat infected forage grasses. However, effects of this
pathogen on whole-plant fitness remain largely unknown. Claviceps
species are likely involved in mutualistic interactions with insect vectors.
During infections, C. paspali translocates sugars from its host grasses,
producing sugar-rich exudates (laden with spores) that may provide a major food
source to the hundreds of species of diverse insects in several orders that
visit infected grass inflorescences and disperse fungal spores. Some of the
insects attracted to infected inflorescences are capable of spreading fungal
spores, thereby potentially spreading the disease. Thus, the fungal pathogen Claviceps
paspali and its insect vectors are likely mutualists.

Other fungal species, including Fusarium heterosporum, may exploit the
mutualism between C. paspali and its insect vectors. F. heterosporum
grows on the exudates produced by C. paspali, thereby potentially
affecting fitness of the pathogen while also decreasing the potential reward
reaped by insects that visit inflorescences infected by both fungi. F.
heterosporum may also change the chemistry of the system, affecting insect
attraction to infected inflorescences or fitness of insects that feed on the
exudates. Further, several species of plant-associated fungi live inside the
leaves and stems of Paspalum species that are hosts of C. paspali,
with unknown effects on fitness of host grasses, C. paspali, F.
heterosporum, or insect vectors. I have submitted a paper coauthored by Dr.
H. E. O’Brien and Dr. A. E. Arnold demonstrating that moths carry spores of F.
heterosporum and fungal endophytes of Paspalum species. As a future
research project, I hope to use this system as a model for insect-vectored
pathogens of plants. Also, I believe that many questions in this system are
conducive to involving undergraduates in research.

(3) What role do
plant-associated fungal viruses play in ecological communities?

Virtually all plant (and
perhaps animal) species harbor pathogenic or mutualistic fungi in their
tissues. Viruses of these fungi have the potential to affect parasitism by
changing virulence of fungal pathogens or mutualisms by altering host tolerance
to environmental stress. However, almost nothing is known about fungal viruses
in an ecological context. In addition, although the “shotgun” sequencing
approach has illuminated extremely diverse and previously unknown fungi,
bacteria and viruses, it is often difficult to determine the associations between
individual species from these presence/absence data. As part of my postdoctoral
research (part of a large NSF-EPSCoR funded project on plant virus
biodiversity, with Dr. M. J. Roossinck), I am conducting a study of fungal
virus prevalence and diversity in a tall grass prairie ecosystem in Oklahoma.
In this system, fungi in diverse genera are hosts to viruses in more than 8
families. All of the 18 viruses I encountered so far (from a sample of 125
plant-associated fungi) are novel, and some are unrelated to known viruses. I
am currently working to develop a system to study effects of some of these
fungal viruses on their fungal hosts (Alternariaalternata and Stemphylium
solani), and on plants associated with these fungi: the parasitic plant Cuscuta
cuspidata and one of its host plants, Ambrosia psilostachya. I use
several molecular tools, including DNA extraction and polymerase chain reaction
(PCR) to identify the Cuscuta species (Cuscuta species are not morphologically
distinguishable unless they are flowering) and fungal species, sterile
techniques to culture the fungi, and double-stranded RNA extractions and
reverse-transcription PCR to obtain partial sequences of fungal viruses. I also
use phylogenetic tools to determine species identities of fungi, and hope to
use such tools in the future to look at relationships among viruses. I hope to
continue work to address effects of fungal viruses on their host fungi and on
interactions between fungi and their associated plant hosts.